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mm: improve page_lock_anon_vma() comment

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A slightly more verbose comment to go along with the trickery in
page_lock_anon_vma().

Signed-off-by: Peter Zijlstra <[email protected]>
Reviewed-by: KOSAKI Motohiro <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: David Miller <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Russell King <[email protected]>
Cc: Paul Mundt <[email protected]>
Cc: Jeff Dike <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Namhyung Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Peter Zijlstra 2011-05-24 17:12:08 -07:00 • committed by Linus Torvalds
commit 6111e4ca68
1 changed files with 16 additions and 2 deletions
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18
mm/rmap.c
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@ -320,8 +320,22 @@ void __init anon_vma_init(void)
}
/*
* Getting a lock on a stable anon_vma from a page off the LRU is
* tricky: page_lock_anon_vma rely on RCU to guard against the races.
* Getting a lock on a stable anon_vma from a page off the LRU is tricky!
*
* Since there is no serialization what so ever against page_remove_rmap()
* the best this function can do is return a locked anon_vma that might
* have been relevant to this page.
*
* The page might have been remapped to a different anon_vma or the anon_vma
* returned may already be freed (and even reused).
*
* All users of this function must be very careful when walking the anon_vma
* chain and verify that the page in question is indeed mapped in it
* [ something equivalent to page_mapped_in_vma() ].
*
* Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
* that the anon_vma pointer from page->mapping is valid if there is a
* mapcount, we can dereference the anon_vma after observing those.
*/
struct anon_vma *page_lock_anon_vma(struct page *page)
{